April Media Highlights: The Geological Society of America Bulletin

Boulder, Colo. - The April issue of the GEOLOGICAL SOCIETY OF AMERICA BULLETIN includes a number of potentially newsworthy items. Please discuss articles of interest with the authors before publishing stories on their work, and please make reference to the GSA BULLETIN in stories published. Contact Ann Cairns for copies of articles and for additional information or assistance.

Widespread evaporitic rocks (gypsum and halite) formed in a short time interval at the end of the Miocene (around 6 million years ago) in the Mediterranean area. Shallow-water evaporites are sandwiched between sediments formed in far deeper marine waters. The complete desiccation of the Mediterranean basin and its transformation into a giant saline is commonly envisaged to explain such unusual rock succession. Dramatic palaeoenvironmental changes with sea-level fall and subsequent rise in the order of more than 1000 meters are usually implied in what has been called the Mediterranean "Messinian salinity crisis."

A study carried out in the Apennines (northern Italy) documents that the vertical superposition of shallow-water evaporites to deeper-water deposits was indeed caused by large-scale submarine sliding processes. These processes were triggered by a Mediterranean-scale phase of tectonic deformation. Local transformation of slides into subaqueous gravity flows and intense subaerial erosion of uplifted shallow-water evaporites led to the accumulation of detritic evaporites into deepest basins. This study suggests that the amplitude of sea-level change associated with the onset of the "Messinian salinity crisis" was less pronounced than previously thought. Moreover, a more effective role of tectonics in controlling this event can be envisaged.

A number of 200 million-year-old elongate basins lie along the east coast of North America. One of them, the Birdsboro basin in the Mid-Atlantic region, accumulated more than 7 kilometers of gravels, sands, silts, and muds. Four major rivers brought sediment from the northwest, and a complex of smaller streams carried sediment from the southeast. These sediments formed regional alluvial fans next to the river mouths, and were deposited in playas and lakes farther from the source areas.

In the North American Midcontinent, rocks of Precambrian ages experienced extensive chemical reaction with potassium-rich fluids. The major product of such reaction is in the form of a potassic mineral: K-feldspar. Determining the condition under which this reaction occurred will help us to learn more about large-scale brine movement, ore formation, and paleogeography of the Mid-continent. This paper applies isotopic dating, study of fluid inclusions trapped in the K-feldspar mineral, and stable isotope analysis to study the potassic reaction. This study revealed at least two episodes of K-feldspar, an early episode of Late Ordovician to Early Devonian age (446 Ma to 395 Ma), and a late episode of Late Devonian to Early Mississippian age. K-feldspar of the late episode most likely formed from saline waters under relatively high temperatures (~100 °C). However, the origin of the older episode K-feldspar is not conclusive.

The late Miocene-Pliocene Bouse Formation of the lower Colorado River trough, located close to the Colorado Plateau, is now found at elevations up to 550 meters above sea level. The Bouse Formation has variously been interpreted as a marine, estuarine, or lacustrine deposit in the past. A marine or estuarine origin for the Bouse Formation imposes constraints upon the rate and timing of uplift that has taken place since deposition. A lacustrine origin imposes no such constraints. This study has performed stable isotope, minor and trace element concentration, and mineralogical analyses of the basal Bouse Formation carbonate in order to help constrain the depositional origin. Although the results are somewhat equivocal, the majority of the data appears to be most consistent with a lacustrine origin for the basal Bouse Formation carbonate, in agreement with the interpretation of Spencer and Patchett (1997). This implies that the present-day elevations of the Bouse Formation place no constraints upon the rate and timing of uplift of the Colorado Plateau.

Early Holocene delevelling and deglaciation of the Cumberland Sound region, Baffin Island, Arctic Canada
Michael R. Kaplan, Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Campus Box 450, Boulder, Colorado 80309-0450, USA, and Department of Geology and Geophysics, University of Wisconsin, 1215 West Dayton Street, Madison, Wisconsin 53706, USA, and Gifford H. Miller, Institute of Arctic and Alpine Research and Department of Geological Sciences, University of Colorado, Campus Box 450, Boulder, Colorado 80309-0450, USA. Pages 445-462.

The eastern Canadian Arctic is a key part of the world for gaining a better understanding of the former Laurentide Ice Sheet, which covered much of North America, including how it interacted with the ocean, affecting regional and perhaps even global climate. In the past, specifically for the Cumberland Sound region on the Baffin Island, much of the geologic record was actually attributed to glacial periods a lot older than 25,000 years. Such an ice age history seemed quite different than that of many other areas on Earth that experienced major glacial expansion between 25,000 and 10,000 years ago. After decades of debate, new geologic dating techniques and a multidisciplinary approach are now allowing scientists to converge on a revised ice age history for many areas along coastal Arctic Canada. Researchers now hypothesize that between 25,000 and 10,000 years ago gentle-sloped fast-moving glaciers did indeed fill valleys and marine embayments, however, adjacent uplands remained ice-free or were covered with relatively inactive glaciers. Furthermore, researchers at the Universities of Colorado and Wisconsin have shown that Cumberland Sound in particular contained a stream of ice, similar to those found today in Antarctica, right until the end of the last ice age about 12,000 years ago, and that subsequently Earth's crust and mantle responded quickly after the ice sheet's weight was removed. In addition, the University of Colorado and Wisconsin researchers also confirmed a previous important hypothesis, that there was a limit to how much the great former Laurentide Ice Sheet impacted the eastern Canadian Arctic. Along the northwestern North Atlantic Ocean, the ice sheet evidently did not overrun certain parts of eastern Cumberland Peninsula, and hence the small region is unique, in that very old archives of local environmental and ecosystem activity have been preserved.

Bear Lake valley is a complex graben in a seismically active region of the eastern Basin and Range province. Bear Lake and its surrounding wetlands currently lie in southern Bear Lake valley, but expanded northward and reached higher elevations (up to 25 m above modern) several times during the Quaternary Period. Laabs and Kaufman used radiocarbon, volcanic ash, and improved techniques in amino acid geochronology to date emergent lacustrine and wetland deposits in Bear Lake valley and reconstruct the history of high water events over the past ~1 million years. Despite the proximity of Bear Lake and Great Salt Lake, the timing of high water events in Bear Lake valley does not coincide with highstands of Lake Bonneville. In fact, Bear Lake may have been smaller than present during the last Bonneville cycle, probably due to important geomorphic constraints on the lake that limit its maximum attainable elevation.

The Orinoco Delta of eastern Venezuela covers ~20,000 km2 of largely undeveloped tropical lowland forests and swamps that represent one of the largest tropical wetland systems in the world. Whereas many modern deltas have been significantly impacted by human activity, the Orinoco offers excellent opportunities to study environmental conditions of a major tropical delta in its natural state. Remote-sensing images, shallow cores, and radiocarbon dating of organic sediments provide new information on the Holocene evolution of this remote and little-studied delta. The delta shoreline has prograded 20-30 km during the late Holocene sea-level highstand through a combination of distributary avulsion and mudcape progradation. This style of delta progradation differs markedly from other major deltas because the Orinoco Delta coastal zone receives prodigious amounts of sediment from NW-moving littoral currents that originate from as far away as the Amazon system (~1600 km). Mudcape development and shoreline progradation, coupled with large amounts of direct precipitation, have led to the development of interdistributary peat swamps that cover up to 5,000 km2 of the delta plain. Hence the Orinoco Delta may be an excellent analog for interpreting ancient tropical deltaic peat deposits

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To obtain a complimentary copy of any GSA BULLETIN article, contact Ann Cairns.